1. Field of the Invention
The present invention relates to well tools and more particularly relates to a protection sleeve used to straddle seal bores such as those in tubing hangers, downhole safety valves and nipples, prior to well intervention operations.
2. Description of Related Art
In oil and gas well production, once a well is completed it may be necessary to re-enter the well to service or adjust equipment and optimise the performance of the well. Such re-entry is termed well intervention. During intervention it is necessary to run protection sleeves which cover seal bores within the existing well completion. These seal bores would typically be part of a tubing hanger, downhole safety valve or nipple. Without protection such seals may be damaged as further equipment is run through the completion. Once located in position, the sleeves allow access into the wellbore during the intervention and are then removed for production of the well to recommence.
Current protection sleeves are wireline conveyed or work string conveyed. Both these typically require a dedicated run to set the sleeve and a dedicated run to remove the sleeve. Once the sleeve is in position, the intervention is carried out via a wireline, coiled tubing or drill string. These provide the necessary clearance between the inner surface of the sleeve and the intervention work string.
Recently, new drilling techniques have placed a higher requirement on these protection sleeves. Due to the maturity of North Sea fields, the number of slots available to drill new wells is limited by the aging infrastructure. To address this, the industry has developed new drilling techniques to use the existing wellhead slots. This technique is termed Through Tubing Rotary Drilling (TTRD). The technique involves drilling a new section of the well through the existing well without having to pull the completion first. The point at which the new section of well is to be started is termed the kick off point. A whipstock packer is set at the predetermined location within the well and this is used to divert the drill string in the new direction. Conventional drillpipe, of smaller outer diameter (“OD”), is then run into the well through the existing tubing string until the whipstock is reached. At this stage the milling operation will commence, drilling a window through the existing tubing and casing to create a ‘sidetrack’ where the new well section is drilled.
As with previous intervention jobs a protection sleeve has to be run to prevent damage to the seal bores within the existing well completion. In the TTRD wells, the protection sleeve now has to protect against rotating drillpipe where non-rotating wireline or coiled tubing was previously used. Further as the milling operation takes place large amounts of debris and metal cuttings will need to be circulated past the sleeve.